Post-tensioning techniques are now commonly used in construction of buildings, bridges, nuclear containment structures, water storage tanks, foundations, dams and other concrete structures. Such techniques are generally recognized as having originated when Eugene Freyssinet began using high strength steel wires for post-tensioning or prestressing concrete beams as early as 1928. By 1939 a system of using wedges for anchoring the wires and jacks for use in stressing was prevalent. Advantages from these techniques today accrue in the form of reduced structural depth, water-tight slabs with minimal cracks, control of deflection, longer spans at more economical cost and the ability to cast concrete in place rather than being required to assemble pre-stressed sections.
Post-tensioning of concrete structures is typically accomplished with tendons. Tendons are formed of prestressing steel, which may be wire, high strength bar or steel strands. Steel strands may be enclosed in sheathing to provide corrosion protection and eliminate bonding between the pre-stressing steel and the surrounding concrete. Tendons in which the pre-stressing steel is permanently free to move relative to the concrete are known as unbonded tendons.
Strands may be anchored with anchor devices which restrain a group of strands or which restrain only one strand. The former are known as multistrand anchors while the latter are known as monostrand anchors.
This invention relates primarily, but is not limited to, monostrand anchorages for unbonded tendons.
An anchor for monostrand tendons typically includes a passage through which the strand passes. The passage usually includes a generally frusto-conical shaped portion. Strands are inserted in passages of appropriate anchors which have been mounted to the structure forms. After concrete has been poured and allowed to set, wedges are interposed against the strands in the frusto-conical cavities and tension is applied to the strands by external means. The wedges restrain the strands from contracting once they have been tensioned. Such an arrangement is shown, for instance, in U.S. Pat. No. 3,956,797 to Brandestini, et al., which is incorporated by reference. A pocket former is typically placed between the anchor and the structural form before pouring to leave a pocket between the anchor and the finished concrete exterior for accessing a strand. After tensioning the pocket is typically filled and finished with grouting material.
Such techniques have proved effective in the many varieties of structures mentioned above. Such an arrangement which leaves the tendon anchorage and end exposed to concrete may prove inadequate in corrosive or aggressive environments, however. Such environments include, for example, parking structures and exposed buildings in coastal areas or post-tensioned pavements which are frequently salted. Other susceptible structures may be underground structures in the vicinity of salted roads, such as building foundations and parking lots.
Although concrete typically protects the prestressing steel in such structures, chloride ions in water or other corrosive elements sometimes reach the steel. Corrosion occurs in the presence of oxygen in such cases. The corrosion of steel forms products which occupy a volume approximately ten times greater than that of the steel which has been corroded. The expansive forces created by this increased volume cause cracking and spalling. This process leads to further exposure of the prestressing steel to corrosive elements and accelerated likelihood of structural failure.
One approach to reducing such corrosion has been to encapsulate the anchorage, including the entire anchor, in an envelope formed of electrially insulating material. U.S. Pat. No. 4,348,844 issued Sept. 14, 1982 to Schupack. et al., for example, which is incorporated by reference, shows such an arrangement. This structure is expensive, however, and fails to take advantage of the fact that sufficient corrosion protection of the anchor casting is normally provided by the alkalinity of the bonded concrete encasement. Such castings in any event have been shown to be much less sensitive than the strand to corrosion. Thus, economy would be achieved in a tendon anchorage which does not require a separate manufacturing step for surrounding the precast anchorage in an envelope, but which allows the anchor instead to be connected in a fluid-tight or fluid-resistant relationship with its adjoining elements in the tendon.